Evolution of sexually dimorphic plumage patterns in estrildid finches.

Pigmentation patterns aid in camouflage and may also serve for communication. To understand the evolution of different types of within-feather pigmentation patterns, we studied the bird family Estrildidae, in which barred and spotted feathers are common. We found that gains and losses of barred or spotted plumage generally occurred simultaneously in both sexes. Sexual dimorphism evolved mostly via sex-specific losses of pigmentation patterns, rather than sex-specific gains, especially for spotted plumage. These results are consistent with evolution by genetic correlation between the sexes, followed by evolution of sex-specific pathways for trait reduction. Evolution by genetic correlation may be common for pigmentation patterns because they are not conspicuous to distant predators, and expressing them should rarely be detrimental. Previous work found correlated evolution of courtship dance and the overall complexity of estrildid pigmentation patterns, but we did not find this association for the extent of pigmentation patterns. Perhaps other properties of pigmentation patterns, and not their extent, evolve together with dance. Furthermore, the extent of barred or spotted plumage was not associated with colour ornamentation, nor with socioecological traits indicating need for camouflage or sexual signalling. This may reflect different and/or multiple functions for these within-feather pigmentation patterns in different species.